What you need to know...
- Maintenance of diploid chromosome complement by mitosis.
- Sequence of events of mitosis, including equator and spindle fibres.
- Cell production by cell culture requires aseptic techniques, an appropriate medium and the control of other factors
Hopefully, you're now really beginning to enjoy your exploration into the detailed workings of the cell. One of the key features of cells which we've not mentioned yet is the fact that they replicate themselves. You already know that cell division is key to the continuation of life and the growth of organisms. It's important therefore that if we're going to study how cells work, that we also learn about how they divide. Cell division is also a key part of modern applied biological techniques, such as in therapeutics and biotechnological industries. In this topic we'll start off with the mechanisms of cell division then learn more about these cell culture techniques.
As you know, the nucleus of cells contains the cell's chromosomes. Chromosomes consist of DNA which carry the code for all of the functions of the cell - we'll discuss this in more detail in the next topic. A key feature of chromosomes is the number of them in a cell's nucleus. Each species has a specific fixed number of chromosomes known as its chromosome complement. Human cells for example have a chromosome complement of 46. In other words, all of your body cells have 46 chromosomes, 23 of which you inherited from your mother and 23 from your father. Horses have a chromosome complement of 46, whereas oak trees have a chromosome complement of 12.
Another way of describing the number of chromosomes in a cell is whether it is diploid or haploid. Diploid cells have the full chromosome complement, whereas haploid cells have half the chromosome complement of that species. Which cells would be haploid? The sex cells (sperm and egg in humans) would be haploid to allow for the new offspring to have the species' chromosome comlement.
For most species, it's crucial that the number of chromosomes remains constant as cells divide. Mitosis is the mechanism of cell division which results in two cells which have the same chromosome complement as demonstrated by the following diagram which summarises these processes in humans:
So, mitosis is the mechanism by which a diploid cell divides to produce two diploid daughter cells which have the same chromosome complement as the parent cell. But, how then does this process work? In order to help us understand and discuss the mechanisms in involved in mitosis we have divided them into a sequence of named stages. You need to be able to describe each of the stages of mitosis, but you are not required to know their names - although you will encounter them a lot if you're looking for more information on mitosis on the Internet (which of course you will because you're a good student who wants to do well!)
The following table shows the sequence of events of mitosis stage by stage. A diagram is included for each of the described stages. For the sake of simplicity the chromosome complement if the cells in the diagram is four - two matching pairs.
|Before mitosis the chromosomes are long thin structures in the nucleus which are not visible even under a microscope.|
|Before mitosis can begin each chromosome must replicate to form two identical chromatids joined at the centromere. At the beginning of mitosis the chromosomes condense and become visible and the nuclear membrane then begins to break down.|
|The chromosomes then line up along the equator of the cell. Spindle fibres from each of the two poles of the cell join to each of the chromosomes at the centromere.|
|The spindle fibres pull the chromatids apart to opposite ends of the cell. Each of the separate chromatids are now described as chromosomes.|
|The nuclear membranes reform and the cytoplasm divides.|
|Two daughter cells have been formed which are diploid - they have the same chromosome complement as the parent cell.|
There's more on the stages of mitosis on BBC Bitesize, Crash Course goes into mitosis in much more detail than you need if you're interested and you can watch mitosis happening in a real plant cell on YouTube.
In addition to cell division occurring naturally within organisms, we have made use of this process in laboratory conditions for industry and medicine. If a cell produces a useful product for humans (such as insulin), we can provide the necessary conditions to allow these cells to divide which will result in an increase in the production. Or, increasingly in medicine we aim to grow entire sections of tissue (such as skin) in order to use in treatment. This process of growing cells in an artificial medium is described as a cell culture.
As a quick Google Image search reveals, there are numerous different ways to culture cells. However whatever type of container the cells are grown in and whether they're grown on a solid or liquid medium, the culture process shares a number of key features. These include:
- Aseptic conditions: Aseptic conditions are crucial when working with microorganisms. This means that steps are taken to ensure that all equipment, surfaces and ingredients have been sterilised to remove any unwanted cells. This is important for two reasons, firstly to prevent unwanted microorganisms from growing in the culture alongside the desired cells. This would result in a contamination which could ruin the culture, and could even prove to be dangerous. We also want to ensure that what we are growing does not escape the culture, particularly when working with potentially harmful microorganisms. Heat and alcohol are routinely used to ensure aseptic conditions are maintained throughout the cell culture process. Check out this video demonstrating aseptic technique.
- Nutrient rich media: When preparing the culture medium, the solid agar jelly or the liquid broth, it is necessary to add the nutrients the cells require to grow and divide. This is normally a general mixture of nutrients for the type of cell being grown, but sometimes a much more specific nutrient mix is used for the species being grown.
- Appropriate factors: Other environmental factors which would affect the growth of the cells need to be controlled during the culture. These include oxygen concentration, temperature and the pH.